Trapped and Accelerated Electrons within a Magnetic Mirror behind a Flux Rope on the Magnetopause

On 6 December 2015, the Magnetospheric Multiscale satellites traversed the southward outflow of a magnetic reconnection on the magnetopause from the outside in. The magnetic structures or the geometries of the field lines inside this outflow are distinguished in detail by using the plasma and magnetic field observations. A flux rope and the magnetic flux piling-up against this flux rope are found embedding in this outflow. The strong magnetosheath magnetic field allows the formation of a magnetic mirror in the flux pileup region. As the Magnetospheric Multiscale satellites approached the flux rope inside this mirror, the pitch angles of electrons converge toward 90 degrees, indicating that the electrons are trapped inside the mirror. The local loss cones of these electrons along the satellite trajectory are estimated. It is found that the triangle-like pitch angle distribution is well embraced by the curves of the estimated local loss cones. The energy spectra inside the mirror show that the electrons are accelerated in the direction perpendicular to the magnetic field. A test particle method is used to reproduce the acceleration process in a fitted 3-D magnetic field model. The results show that the quasi-perpendicular electrons experience the betatron acceleration when drifting into the stronger field region and that the electrons may also experience the Fermi acceleration when their pitch angles enlarge. The mirror structure on the magnetopause provides a unique opportunity to explore the behaviors of the magnetic reconnection associated electrons.